The present invention relates generally to titanium alloy processing and more specifically to a novel high speed microstructural conversion method for converting Ti—AL—4V from lamellar to equiaxed morphology.
Among all titanium alloys, Ti—6Al—4V is the most widely used and accounts for the majority of applications. The mechanical properties of this alloy are very sensitive to its microstructure which, in turn, is significantly influenced by deformation processing. Since the resistance to fatigue crack initiation of equiaxed α+β microstructure is about two times higher than that of lamellar, equiaxed α+β microstructure is preferred for use in rotating components such as turbine disks. But, this requires a conversion process to be performed to convert the microstructure from lamellar to equiaxed.
Microstructural conversion from lamellar to equiaxed α+β is the most critical and time-consuming step in the processing sequence and is conventionally achieved using a series of hot forging steps in the α+β phase field. Since hot working at faster speeds produces microstructural defects such as adiabatic shear bands, cracking, and lamellae kinking, conventional forging speeds are limited to a strain rate of about 0.1/s. In addition, the occurrence of undesirable microstructures at slow speeds demands precise temperature control over a narrow range, which is difficult to achieve under complex manufacturing conditions.
The prior art conversion method in common use today consists of several steps of extensive deformation in the α+β temperature range to obtain desired microstructure and product dimensions. In view of the occurrence of microstructural instabilities that result from processing at high speeds (e.g. cracking, adiabatic shear banding, lamellae kinking etc.), this prior art conversion method is performed at slow speeds using machines such as hydraulic presses. Temperature control is critical and the occurrence of strain induced porosity has been a major problem because the temperature often falls below a safe limit during processing. Deformation at high temperatures, close to the β transus (α+β→β transformation temperature), on the other hand, will result in a β transformed microstructure and the purpose of conversion will be lost. Therefore, the conventional, prior art conversion process is not only slow from a manufacturing view point, it also demands precise temperature control over a narrow range which is difficult to achieve in large components.
A need exists therefore for an improved process for microstructural conversion which is not only faster than the prior art process but also provides good microstructural control. Such a process would provide improved conversion results and be simpler to implement.